Conventional syringes have a barrel and a closely fitting piston which draws fluid into the barrel via a needle in front of the barrel. Fluid is drawn into the barrel through the needle, the air is expelled and an injection is made by depressing the plunger. Many of these medical devices have been designed to retract the needle by various mechanisms because of the continuing danger of exposed needles contaminated with infectious agents. With the increase of dangerous communicable diseases such as AIDS, it has become critical to eliminate needle stick injuries to medical personnel. Intensive efforts have been directed to development of retractable syringes which are safe, effective and practical, which can be mass produced at low cost.
Seemingly ignored in all this activity is the smaller but still significant group of syringes which employ a pre-filled cartridge of fluid medication and a double ended hypodermic needle communicating with the cartridge for injecting the contents of the cartridge. The pre-filled syringe cartridges are referred to as "carpules". They are typically cylindrical tubes with a puncturable membrane in front and a piston seal at the rear which is pressed forward by some form of plunger. The most common of these are the carpule syringes used by dentists in freezing the gums of their patients prior to their performing dental work on their teeth. Typically, the syringe enclosures with which such pre-filled syringe cartridges are used are not easily capable of retracting the needle into a protective enclosure to avoid inadvertent and potentially harmful needle sticks. Consequently, most syringes used for this purpose by dentists have a fixed needle which must be sheathed.
The relatively few attempts that have been made to produce a retractable needle syringe have produced results not altogether satisfactory. Weltmnan, U. S. Pat. No. 3,306,290; Sullivan, U. S. Pat. No. 5,330,430; and Haber, U.S. Pat. No. 4,820,275 among other things suffer from the deficiency that the device is necessarily much longer than the stroke the plunger itself would require in order that the outer shell house both needle and cartridge. Stanners, U. S. Pat. No. 5,330,440, although it doesn't suffer from the length deficiency, employs special thread engaging plugs in both ends of the carpule and plunger. These threaded connections must be mechanically connected together to withdraw the needle to the back of th e special carpule. Retraction is done manually by disengaging a catch.
Although the needles can be retracted, these devices do not provide instantaneous retraction of the needle automatically at the end of an injection by further depression of the plunger while the needle is still in the patient's tissue. A slow controlled manual retraction of the needle is undesirable. Unintended movement of the syringe could damage tissue. Carpule syringe devices that would meet the above objections and which enable instantaneous retraction by continuation of the same motion used for the injection would be a significant improvement. These and other objects are the subject of the present invention.